1. Field of the Invention
The invention is a process for forming a coating on aluminum and a composition for use in the process. The coating composition comprises (a) phosphate ion, (b) an element selected from the group consisting of Zr, Ti, Hf, and Si based on fluorozirconic, fluorotitanic, fluorohafnic, and fluorosilicic acid and a polyphenol composition and water. The composition when contacted with clean aluminum forms an inorganic--organic conversion coating in one step.
2. Statement of Related Art
It is well known that the corrosion resistance and paint adhesion properties of an aluminum substrate can be improved by forming a chromate conversion coating on the substrate. The chromium containing coating is formed by contacting the substrate with an aqueous conversion coating composition containing hexavalent chromium ions, phosphate ions, and fluoride ions. In recent years, there has been a growing concern about pollution of the environment with toxic chemical materials. Hexavalent chromium can cause problems if discharged into waterways because of its strongly oxidizing character. As a result, conventional chromate conversion coating processes require extensive waste treatment procedures to eliminate possible harmful effects resulting from the discharge of hexavalent chromium. Treatment of the hexavalent chromium wastes results in increased costs and the difficulties with disposing of the waste solutions.
Chromium-free coatings for aluminum are known. U.S. Pat. No. 4,148,670 discloses an acidic aqueous conversion coating solution for aluminum which comprises zirconium or titanium, a phosphate ion, and available fluoride. These solutions are prepared generally from ammonium salts of fluorozirconic acid or fluorotitanic acid, phosphoric acid, and hydrogen fluoride. The solution is used to contact an aluminum substrate in a pH range of about 1.5 to about 4 and forms a conversion coating thereon.
British patent application 2,165,165 discloses a coating process for aluminum. The process comprises contacting a cleaned aluminum surface with an aqueous acidic treatment composition to form a coating thereon, rinsing the coated metal surface with water and contacting the rinsed metal surface with a post-treatment solution and drying the metal surface.
The aqueous acidic treatment conversion coating solution comprises dissolved metal ions selected from the group consisting of hafnium, zirconium, titanium and mixtures thereof, phosphate ions, fluoride ions, vegetable tannin compound and a sequestering agent. After contact with the acidic conversion coating solution, the aluminum surface is then rinsed and contacted with a solution of a Mannich adduct of polyalkenyl phenol. The process is a three-step process and requires considerable manipulation of the aluminum being coated.
U.S. Pat. No. 4,191,596 discloses a method and composition for coating aluminum. Aluminum is coated by contacting a clean aluminum substrate with a mixture consisting essentially of (a) polyacrylic acid or esters thereof and, (b) at least one acid selected from the group consisting of fluorozirconic, fluorotitanic and fluorosilicic acids. The coating process is a single step process and is essentially chromium and phosphate-free. The composition has achieved commercial success.
U.S. Pat. No. 4,136,073 discloses a process for treating aluminum by contacting the aluminum with an acid composition consisting essentially of a stable organic film forming polymer and a soluble titanium compound. The film forming polymer exemplified is a polyacrylic ester. Fluoride and phosphate are not critical to the composition. One treating composition exemplified contains fluoride and phosphate, the treating composition of Example 2 does not disclose phosphate as a component.
In view of the difficulties associated with using chromium conversion coatings, it is desirable to have a non-chromium conversion coating which provides an organic finish coated aluminum with paint adhesion and corrosion resistance, substantially equivalent to organic finish coated chromate treated aluminum. It would be desirable to achieve the aluminum coating in a single step.